1. Field of the Invention
The present invention relates to a nitride semiconductor used for manufacturing, for example, a semiconductor laser device, a semiconductor device using the nitride semiconductor, and a method of manufacturing the nitride semiconductor and the semiconductor device.
2. Description of the Related Art
In recent years, attention has been given to Group III-V compound semiconductors as device materials because of their various properties. Specifically, Group III-V material systems are direct transition type, and their forbidden bandwidths range from 1.9 eV to 6.2 eV, so only the material systems can emit light in a wide range from a visible region to an ultraviolet region. Therefore, the development of the material systems as materials of semiconductor light-emitting devices such as semiconductor lasers and light-emitting diodes (LEDs) has been actively pursued. Moreover, in addition to wider forbidden bandwidths, as their saturated electron velocities and dielectric breakdown electric fields can be expected to become higher, so in respect of high-temperature operation, high-rate switching operation, large current operation and so on, applications of the material systems as devices operated in an region where conventional Si material systems or GaAs material systems are inoperable in principle have been researched.
Gallium nitride semiconductors such as GaN, AlGaN and GaInN among the Group III-V compound semiconductors are material systems which are increasingly applied to devices, and such semiconductor devices are conventionally manufactured by laminating a gallium nitride semiconductor film on a surface of a crystal substrate or a crystal film. The crystal substrate (or the crystal film) is preferably made of a bulk crystal of a gallium nitride compound, although it is difficult to manufacture the bulk crystal of this kind. Therefore, as a matter of fact, a gallium nitride compound is grown on a substrate made of sapphire (α-Al2O3), silicon carbide (SiC) or the like through epitaxial growth in the majority of cases.
However, there are large differences in lattice mismatch and thermal expansion coefficient between a substrate material such as sapphire and the gallium nitride compound, and in order to ease the distortions, lattice defects such as dislocation occur in a layer made of the gallium nitride compound. A portion where the lattice defects exist becomes the center of a non-radiative recombination which emits no light even though electrons and holes are recombined or a point where a current leaks, thereby resulting in impaired properties of the semiconductor device.
Therefore, crystal growth to remove defects from the gallium nitride compound has been studied, and growth techniques using a property that in a crystal grown in a lateral direction with respect to a seed crystal, which is a base for growth, that is, in a horizontal direction with respect to a surface of a layer to be formed, fewer dislocations derived from the seed crystal exist is being applied to GaAs crystals and GaN crystals at present.
For example, in Japanese Unexamined Patent Application No. Hei 10-312971, the following method is adopted. A GaN layer is formed on a sapphire substrate, and a growth inhibition layer made of SiO2 (silicon dioxide) is formed on a surface of the GaN layer, then a GaN crystal is grown from a GaN surface as a base which is exposed via the growth inhibition layer. According to the method, the growth of dislocations can be inhibited by the growth inhibition layer, so dislocations which penetrate the crystal to reach to a surface of the crystal (so-called threading dislocations) are reduced. However, in an aperture portion of the growth inhibition layer, dislocations passing through the aperture portion and then penetrating the crystal exist, so dislocations or defects partially increase in a region above an aperture portion of the gallium nitride semiconductor layer.
Moreover, there is the following method as a technique of another type. For example, a large number of seed crystal portions are formed from a GaN layer through patterning, and crystal growth takes place in a lateral direction from the seed crystal portions as bases, and then, crystals grown in a lateral direction meet one another between the seed crystal portions. However, even in this method, dislocations propagate toward top surfaces of the seed crystal portions, so regions directly above the seed crystal portions become regions where a large number of dislocations or defects locally exist. Therefore, even if these methods are used, surface defects in the gallium nitride semiconductor on the substrate cannot be sufficiently reduced.
Further, lateral growth in these methods is not perfect selective growth, so a crystal is grown in a lateral direction as well as in an upward direction at the same time. Thereby, while the crystal is sufficiently grown in a lateral direction, the thickness of the crystal is increasing more and more, which may result in the occurrence of warpage in the gallium nitride semiconductor layer formed. Therefore, the same inventors as those of the present invention have previously attempted to grow a gallium nitride semiconductor at a higher temperature than before so that lateral growth predominantly take place to make the layer thickness thinner. The higher the growth temperature is, the higher directivity of a growth direction increases, and thereby lateral growth is promoted, but in this case, defects called hillocks may occur on the layer surface. The hillocks are crater-like projections with a diameter ranging from 70 μm to 100 μm and a height of about 0.7 μm, and experiments have revealed that the hillocks have a tendency to occur mainly in a region directly above the seed crystal portion (or the aperture portion of the growth inhibition layer). Defects occur in a semiconductor layer grown on the hillocks, which may result in impaired properties of a semiconductor device manufactured. In a semiconductor laser, when a laser stripe is formed on the hillocks, there is a decline in reliability of the laser such as a decline in a laser static property and a shortening of the life of the laser.